Osteochondropathy in the bones of the leg - often referred to by the
unspecific term of "leg weakness" - remains a major cause of losses of
commercial poultry , including turkeys. The diagnosis and study of these
disease entities and the differentation between physiologic and pathologic
growth are rendered difficult by lack of a process study of the development of
physiologic bone parameters in the osteochondral junction of the epiphyses of
the hind limb over a full period of growth. The goal of the present study was
to illustrate the growth and differentiation processes in the growth plates of
the long bones of the hind limb over the course of a full growth period. The
study group included a total of 94 white tom turkeys of the double hybrid line
B.U.T. Big 6 ranging in age from one day to 147 days. These animals were
obtained from a conventional turkey farm. The study documents the development
of macroscopic parameters such as body weight, bone length, wet weight of the
femur, and the craniocaudal diameter of the femoral and tibiotarsal diaphysis.
Special attention was also devoted to observing the histomorphometric and
histologic changes in the growth plates and their various zones and to the
development of the vascular system of the proximal tibiotarsal growth plate.
The size and resorption of the embryonic cartilage cone were also subject of
histologic evaluation. This involved preparation of approximately 1900
transverse paraffin sections of the ends of the bones treated with hematoxylin
and eosin stain. These sections were then respectively measured or evaluated
with the aid of anautomatic image analysis system. The histologic findings
were also compared with radiographic findings. Growth plate closure and the
development of the ossification centers and embryonic cartilage cone were
evaluated in 564 radiographs in two planes. The progression of the
relationship between the area of medullary canal and the area of the cortex
was demonstrated in 188 radiographs of rings of the femoral and tibiotarsal
diaphyses with the aid of automatic image analysis system. Additionally, the
femoral diaphyseal sections of the group from the respective week were
incinerated and the mineral content of the ash analyzed for calcium and
phosphorus. Bone density measurements of proximal femoral metaphysis were also
obtained. The results show that body weight, bone lengths, craniocaudal
diameter of the femoral and tibiotarsal diaphyses, cross-sectional area of the
femoral and tibiotarsal diaphysis, and the wet weight of the femur all develop
at relatively uniform rates. The progession of these parameters describes a
more or less distinct sigmoidal curve. The physiologic embryonic cartilage
cones were also evaluated. Histologic and radiographic examinations
demonstrated their presence in all of the one-day-old bone end specimens, and
they were completely absorbed between the second and third week of life. The
growth plate specimens that were evaluated exhibited a uniform histologic
appearence with respect to their architecture and arrangement of their zones.
They all attained their greatest size in the sixth week of life. There were
differences in the thickness of the growth plate. A distinction has to be made
between type 1 growth plates and type 2 growth plates. The former are
characterized by their relatively constant thickness, which ranges between
3000 and 4000 µm and late continuous decreases. The femoral growth plates are
of this type. In contrast, the growth plates of the proximal tibiotarsus and
proximal tarsometatarsus are type 2 growth plates. These typically exhibit a
rapid increase in thickness to values ranging between 8500 and 8800 µm
followed by rapid reduction. The growth plate of the distal tibiotarsus
represents an intermediate form. Concerning the growth plate the proliverative
zone in proportion to the hypertrophic one is not stady. All growth plates
reach their maximum thickness in the 7 th week of life. No anastomoses were
ever observed between the penetrating epiphyseal vessels supplying the
proliferation zone of the growth plate and metaphyseal vascular system, which
is responsible for matrix calcification, cytolysis and resorption of
chondrocytes, and ossification. Reduction of the growth plate is initially
attributable to a decrease in the thickness of the hypertrophic zone and to a
decrease in the number and luminal diameter of the penetrating epiphyseal
vessel canals. The process ends with the complete resorption of the growth
plate. This occurs in the proximal femur in the 19th week of life, in the
distal femur in the 18th week, in the proximal tibiotarsus in the 19th (20th)
week, in the distal tibiotarsus in the 14th week, and in the proximal
tarsometatarsus in the 18th week respectively. A total of five ossification
centers were demonstrated in both epiphysis of the tibiotarsus and the
proximal tarsometatarsus. The solitary ossification center occuring in the
proximal tibiotarsus could only be demonstrated on radiographs in all animals
in the fourth week of life. Its relatively late emergence and its location at
the insertion of the patellar ligament suggest that it is a secondary center.
In contrast, the two condylar ossification centers in the distal tibiotarsus
and in the proximal tarsometatarsus (already present in some of the animals on
the first day of life) may be regarded as primary centers as they arise from
tarsal bones. Also visible on the first day of life, the so-called theird
ossification center in the distal epiphysis of the tibiotarsus lies in the
middle of the insertions of three ligaments and is accordingly a secondary
center. Between the fifth and eleventh weeks of life, the density of the
proximal femoral metaphysis increases by a factor of 3.5. In contrast,
incineration and mineral analysis of the femoral diaphysis show maximum
increases only up to the seventh week of life. The data presented in this
paper on the physiologic development of the bones of the hind extremity of the
turkey should aid in the early detection and further study of
osteochondropathy. Growth phases characterized by rapid changes and intensive
remodelling, which are therefore associated with increased risk to the
immature skeleton, are worked out and taken into consideration.